

By your rational a police agent without a warrant could walk by and say “hello”, plant a listening device, then record your conversation because he said hello at the start.
No. In that situation a third party inserted themselves into your conversation entirely of their own volition.
This is like you walking up to someone that’s streaming/vlogging in public, beginning an unrelated conversation in front of them; then you getting upset that they are recording the conversation that you began in their presence. Even if you weren’t aware they were streaming; you were the one that inserted yourself into that situation. They didn’t walk up to/join you; you made them a party by bringing the conversation to them.
A really big part of these types of legal situations is ‘reasonable expectation of privacy’. The people inside a vehicle are all pretty close together and obviously going to be able to hear the conversions that are happening. It’s unreasonable to expect the driver who’s head is ~3 feet from you isn’t privy to your conversation.
This is true.
What you don’t understand is that a person does not have to be actively speaking or being directly spoken to in order to be a part of a conversation. Simply being present, with the other participants fully aware of your presence while continuing to converse makes you part of their conversation and thus a party able to consent to it’s recording.
The key there is that the other participants are aware of your presence. You’re not hiding around a corner, listening in unbeknownst to them; the people conversing are entirely aware that you are present and likely listening.